Beyond the stars: Linking H α sizes, kinematics, and star formation in galaxies at <i>z</i> ≈ 4−6 with <i>JWST</i> grism surveys and <scp>geko</scp>

ABSTRACT Understanding how galaxies assemble their mass during the first billion years of cosmic time is a central goal of extragalactic astrophysics, yet joint constraints on their sizes and kinematics remain scarce. We present one of the first statistical studies of the $\mathrm{H}\alpha$ size–mass relation at high redshift with a sample of 213 galaxies at spectroscopic redshifts of $z\approx 4-6$ from the FRESCO and CONGRESS NIRCam grism surveys. We measure the $\mathrm{H}\alpha$ morphology and kinematics of our sample using the novel forward modelling Bayesian inference tool geko, and complement them with stellar continuum sizes in the rest-frame far ultraviolet (FUV), near ultraviolet (NUV), and optical, obtained from modelling of imaging data from the JADES survey with Pysersic. At $z\approx 5$, we find that the average H $\alpha$ sizes are larger than the stellar continuum (FUV, NUV, and optical), with $r_{\rm e, \mathrm{H}\alpha } = 1.17 \pm 0.05$ kpc and $r_{\rm e,cont} \approx 0.9$ kpc for galaxies with $\log (M_{\star } \rm [{\rm M}_{\odot } ]) = 9.5$. However, we find no significant differences between the stellar continuum sizes at different wavelengths, suggesting that galaxies are not yet steadily growing inside–out at these epochs. Instead, we find that the ratio $r_{\rm e, \mathrm{H}\alpha }/r_{\rm e, NUV}$ increases with the distance above the star-forming main sequence ($\Delta \rm MS$), consistent with an expansion of H $\alpha$ sizes during episodes of enhanced star formation caused by an increase in ionizing photons. As galaxies move above the star-forming main sequence, we find an increase of their rotational support $\text{v}/\sigma _0$, which could be tracing accreting gas illuminated by the $\mathrm{H}\,\alpha$ emission. Finally, we find that about half of the elongated systems ($b/a\lt 0.5$) are not rotationally supported, indicating a potential flattened/prolate galaxy population at high redshift.